Without limiting the scope of the present disclosure, its background will be described with reference to perforating a cased wellbore with a perforating gun assembly, as an example.
After drilling each section of a wellbore that traverses various subterranean formations, individual lengths of relatively large diameter metal tubulars are typically secured together to form a casing string that is positioned within the wellbore. In addition to providing a sealing function, the casing string provides wellbore stability to counteract the geomechanics of the formations such as compaction forces, seismic forces and tectonic forces, thereby preventing the collapse of the wellbore wall. The casing string is generally fixed within the wellbore by a cement layer that fills the annulus between the outer surface of the casing string and the wall of the wellbore. For example, once a casing string is located in its desired position in the wellbore, a cement slurry is pumped via the interior of the casing string, around the lower end of the casing string and upward into the annulus. After the annulus around the casing string is sufficiently filled with the cement slurry, the cement slurry is allowed to harden, thereby supporting the casing string and forming a substantially impermeable barrier.
To produce fluids into the casing string or inject fluids into the formation, hydraulic openings or perforations must be made through the casing string, the cement and a short distance into the formation. Typically, these perforations are created by detonating a series of shaped charges that are disposed within the casing string and are positioned adjacent to the desired formation. Specifically, one or more charge carriers are loaded with shaped charges that are connected with a detonating cord. The charge carriers are then connected within a tool string that is lowered into the cased wellbore at the end of a tubing string, wireline, slick line, electric line, coil tubing or other conveyance. Once the charge carriers are properly positioned in the wellbore such that the shaped charges are adjacent to the interval to be perforated, the shaped charges are detonated. Upon detonation, each shaped charge generates a high-pressure stream of metallic particles in the form of a jet that penetrates through the casing, the cement and into the formation with the goal of forming an effective communication path for fluids between the reservoir and the wellbore.